Nonbasic lands will lose any other land types and abilities they had. They will gain the land type Mountain and gain the ability “: Add to your mana pool.”

3/14/2017

Blood Moon’s effect doesn’t affect names or supertypes. It won’t turn any land into a basic land or remove the legendary supertype from a legendary land, and the lands won’t be named “Mountain.”

3/14/2017

If a nonbasic land has an ability that causes it to enter the battlefield tapped, that ability will still function. For example, if Blood Crypt is entering the battlefield, its controller may pay 2 life to have it enter untapped. Regardless of this choice, it will be only a Mountain and not a Swamp. The same is also true of any other abilities that modify how a land enters the battlefield or apply “as” a land enters the battlefield, such as the first ability of Cavern of Souls.

3/14/2017

If a nonbasic land has an ability that triggers “when” it enters the battlefield, it will lose that ability before it triggers.